Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a specimen. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
Metrology processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. A number of optical metrology based techniques including scatterometry and reflectometry implementations and associated analysis algorithms are commonly used to characterize critical dimensions, film thicknesses, composition and other parameters of nanoscale structures.
Traditionally, scatterometry measurements are performed on targets consisting of thin films and/or repeated periodic structures. During device fabrication, these films and periodic structures typically represent the actual device geometry and material structure, or an intermediate design. As devices (e.g., logic and memory devices) move toward smaller nanometer-scale dimensions, characterization becomes more difficult. Devices incorporating complex three-dimensional geometry and materials with diverse physical properties contribute to characterization difficulty. For example, modern memory structures are often high-aspect ratio, three-dimensional structures that make it difficult for optical radiation to penetrate to the bottom layers. In addition, the increasing number of parameters required to characterize complex structures (e.g., FinFETs), leads to increasing parameter correlation. As a result, the parameters characterizing the target often cannot be reliably decoupled with available measurements. In another example, opaque, high-k materials are increasingly employed in modern semiconductor structures. Optical radiation is often unable to penetrate layers constructed of these materials. As a result, measurements with thin-film scatterometry tools such as ellipsometers or reflectometers are becoming increasingly challenging.
In response to these challenges, more complex optical tools have been developed. For example, tools with multiple angles of illumination, shorter and broader ranges of illumination wavelengths, and more complete information acquisition from reflected signals (e.g., measuring multiple Mueller matrix elements in addition to the more conventional reflectivity or ellipsometric signals) have been developed. However, these approaches have not reliably overcome fundamental challenges associated with measurement of many advanced targets (e.g., complex 3D structures, structures smaller than 10 nm, structures employing opaque materials) and measurement applications (e.g., line edge roughness and line width roughness measurements).
Another response to these recent challenges in semiconductor manufacturing has been the adoption of x-ray metrology for measurements including film thickness, composition, strain, surface roughness, line edge roughness, and porosity.
In some examples, traditional x-ray based metrology tools lack the ability to adapt to a wide range of measurement system configurations. This limits the ability to select a measurement system configuration tuned for a specific metrology application. In some of these examples, the lack of machine flexibility and machine parameter optimization results in a measurement system that is unable to resolve particular parameters of interest. In other examples, a measurement system is unable to achieve the required measurement precision for particular parameters of interest in a reasonable amount of measurement time.
In some other examples, x-ray metrology tools include a broader range of measurement system configurations. However, the use of the complete range of available system parameter values may result in excessively long measurement times. In one example, X-ray scattering measurements are performed on periodic targets. Typically, the intensity of the X-ray diffraction orders is detected at a set of evenly spaced angles orthogonal to the direction of periodicity. Such spacing may be too coarse and the measurement system is unable to resolve the particular parameters of interest. Alternatively, a closer spacing may be employed, but the measurement system is unable to achieve the required measurement precision in a reasonable amount of measurement time.
Future X-ray based metrology applications present challenges due to increasingly small resolution requirements, multi-parameter correlation, increasingly complex geometric structures, and increasing use of opaque materials. Thus, methods and systems for improved selection of X-ray based metrology system parameters are desired.